Potentiometer



March 6, 1956 c. s. SlVlL 2,737,561

FOTENTIOMETER Filed Oct. 10, 1955 INVENTOR.

United States Patent POTENTIOMETER Cecil S. Sivil, East Orange, N. 1., assignor to Baker & Co., Inc., Newark, N. 1., a corporation of New Jersey Application October 10, 1955, Serial No. 539,350

6 Claims. (Cl. 201-42 The present invention deals with potentiometers and more particularly with precision potentiometers.

This application is a continuation-in-part of application Serial No. 303,280, filed August 8, 1952, now abandoned.

Precision potentiometers, especially those of the low torque type, comprise essentially a winding of high resistance wire and a contacting movable brush member or slider whereby movement of the brush member or slider is capable of continuously dividing or otherwise changing the voltage output of the voltage applied across the windmg.

The relationship between the brush or slider and winding is critical. The linkage operable to move the brush must have a minimum of friction in order to provide low torque movement capable of smoothly and evenly moving the brush over the winding for high accuracy. The composition of the winding and brush must provide for the combined properties of high resistance, low temperature coefiicient or resistance, low contact resistance; and the wire winding must have high tensile strength.

Although various precious metal alloys may be lused for the brush and winding members, or alloys of precious metal and a base metal, such as platinum-iridium or platinum-tungsten, there are differences in each of the above noted requirements which, even though of seemingly small magnitude, affect the operation of the potentiometer when employed in combination to provide the desired result. For example, in the movement of the brush over the coil or winding, the resolution between the individual windings must be substantially finite. Therefore, it is possible that a substantially small difference between one of the properties of the alloys"- 'em ployedother properties being substantially identicalincreases or decreases the potentiometer accuracy. In case of precision measurements, it is apparent that the degree of resolution must be as high as possible in order that a high linearity accuracy may be maintained throughout the movement of the brush over the winding.

It is an object of the present invention to provide an improved precision potentiometer. It is another object of the present invention to provide an improved precision potentiometer substantially free of noise and having high linear accuracy. It is a further object of the present invention to provide a precision potentiometer having excellent wear resistant and tarnish resistant properties for excellent contact between brush and resistance winding. Other objects and advantages of the present invention will become apparent from the drawing and description hereinafter following.

The potentiometer of the present invention consists of a resistance winding 1 and a movable contact or brush member 2 in electrical contact with the said winding. The winding 1 and contact 2 are by way of illustration shown as components of an electrical circuit having conductors 3 and 4 connected across the coil 1 and to a source of voltage E, and conductors 5 and 6 connected to a terminal of the winding 1 and the contact 2 respectively and to the voltage output E1.

The invention consists in the combination of a contact or brush 2 composed of a suitable contact alloy of precious metal, e. g. platinum-20% iridium, or an alloy of precious metal and a base metal wherein the precious metal constitutes a major proportion of the alloy, e. g. platinum in excess of in proportion to the base metal comprising less than 15% molybdenum or tungsten as disclosed in Patent No. 1,407,525, and a resistance wire 1 consisting essentially of at least one metal of the group consisting of palladium and platinum with 1%-l0% chromium.

The specific composition of the resistance wire provides, in combination with the contact member above set forth, a potentiometer having excellent electrical resistance, temperature coefiicient of resistance and tensile strength.

The following table illustrates the desirable properties of the potentiometer resistance wire composed of platinum and/or palladium and 1%-10% chromium:

Temperature Resistance, Tensile Percent Cr ggfig g l Ohms per Strength,

ance C M. p. s. l.

It will be noted from the above table than an increase in the chromium content from 1% to 10% is accompanied by a decrease in the temperature coefficient of resistance and increase in resistance and tensile strength.

Although, generally, the alloys containing from 1% to 3% chromium are satisfactory, the alloys containing from 5% to 10% chromium or the preferred range of 5% to 7% chromium are characterized by an extremely low temperature coefficient to resistance, high resistance and high tensile strength, and high resolution with a consequently high linear accuracy.

The resistance wire of the invention is, furthermore, tarnish proof and highly resistant to wear over long periods of time which contributes to freedom from noise due to the excellent contact between the winding and the brush composed of an alloy having a major proportion of precious metal content.

The combination of brush and winding composed of alloys above set forth is excellently suitable to the application of low torque movement of the brush for the high accuracy and sensitivity desirable in precision potentiometers.

What I claim is:

l. A precision potentiometer comprising a resistance winding and a movable brush member contacting said winding, said winding consisting of a wire composed of platinum containing from 1% to 10% chromium, said brush member composed of an alloy containing precious metal as a major proportion of the alloy.

2. A precision potentiometer comprising a resistance winding and a movable brush member contacting said winding, said winding consisting of a wire composed of platinum containing from 5% to 10% chromium, said brush member composed of an alloy containing precious metal as a major proportion of the alloy.

3. A precision potentiometer comprising a resistance winding and a movable brush member contacting said winding, said winding consisting of a wire composed of platinum containing from 5% to 7% chromium, said brush member composed of an alloy containing precious metal as a major proportion of the alloy.

4. A precision potentiometer according to claim 2 wherein said brush member is composed of 80% platinum and 20% iridium.

5. 1A precision potentiometer according to. claimlh,

wherein said brush member is composed of more than 85% platinum and less than 15% of a metal taken from the group consisting oftungstenand molybdenum.

6. A precision potentiometer comprising :a resistance winding and a movable brush member contacting said winding, said winding consisting of a wire composed of at least one metal of the group consisting \of platinum and palladium containing from 1% to-10% chromium, said brush member composed of an alloy containing 10 precious'metal as a major proportion of the alloy.

.v ,mketerencas.Slitedinth file of. this p tent UNITED STATES PATENTS 2,391,455 Hensel Dec. 25, 1945 2,406,966 Pfeil Sept. 3, 1946 2,537,733 Brenner Jan. 9, 1951 OTHER REFERENCES Vines: The Platinum Metalsand their Alloys, 1941 Thelnternational Nickel Co., 67 Wall St., New York. 

